Lubricant composition

ABSTRACT

Provided is a lubricant composition containing a lubricant base oil and 0.5 to 10 mass % of (A) an ethylene/α-olefin copolymer having a number average molecular weight of 2,800 to 8,000, in which the lubricant base oil used here has a kinematic viscosity at 100° C. of 1.5 to 40 mm 2 /s, a viscosity index of 100 or more, a pour point of −25° C. or lower, and a sulfur content of 0.01 mass % or less, and the lubricant composition has low viscosity and a long metal fatigue life, while being excellent in low-temperature fluidity and oxidation stability, thereby being particularly suitable as a lubricant for automobile transmissions or the like.

TECHNICAL FIELD

The present invention relates to a lubricant composition, and morespecifically, to a lubricant composition having low viscosity and has along metal fatigue life, while being excellent in low-temperaturefluidity and oxidation stability, thereby being particularly suitable asa lubricant for automobile transmissions or the like.

BACKGROUND ART

Energy savings, that is, fuel savings in, for example, constructionmachines, agricultural machines and automobiles have recently becomeurgent matters with the advent of the need to cope with environmentalissues, so an apparatus such as an engine, a transmission, a finalreduction gear, a compressor, or a hydraulic system has been stronglyrequested to contribute to the energy savings. Accordingly, a lubricantto be used in any such apparatus has been requested to reduce thestirring resistance or frictional resistance of the apparatus to alarger extent than a conventional lubricant does.

For example, a reduction in viscosity of a lubricant is one method ofachieving fuel savings in each of the transmissions and the finalreduction gears. Of the transmissions, an automatic transmission orcontinuously variable transmission for automobiles has, for example, atorque converter, a wet clutch, a gear bearing mechanism, an oil pump,and a hydraulic control mechanism, and a manual transmission or finalreduction gear has a gear bearing mechanism. A reduction in viscosity ofa lubricant to be used in any such transmission reduces the stirringresistance and frictional resistance of, for example, each of the torqueconverter, the wet clutch, the gear bearing mechanism, and the oil pump,and improves the efficiency of power transmission, thereby enabling animprovement in fuel efficiency of an automobile.

However, a lubricant with its viscosity reduced is apt to be lost byevaporation particularly when used in a high-humidity environment.Moreover, a problem such as an increase in coefficient of friction of afilm composed of the lubricant arises in association with the reductionin viscosity. As a result, the lubricant has a significantly reducedfatigue life to cause seizing or the like, so a transmission or the likemay undergo malfunctions. In particular, when a phosphorus-based extremepressure agent is blended for improving the extreme-pressure property ofa low-viscosity oil, the fatigue life of the oil remarkablydeteriorates, so it is generally difficult to reduce the viscosity ofthe oil.

For example, a composition obtained by the following procedure has beendisclosed (see, for example, Patent Documents 1 and 2): a conventionaltransmission oil for automobiles is blended with, for example, asynthetic oil-based lubricant base oil and/or a mineral oil-basedlubricant base oil, an anti-wear agent, an extreme pressure agent, ametal-based detergent, an ashless dispersant, a friction adjustor, or aviscosity index improver with the amount of any such additive optimizedso that the composition to be obtained may be able to maintain variousproperties such as a transmission characteristic for a long time period.However, no investigation has been conducted on an influence of any suchcomposition on the fatigue life of a lubricant when the viscosity of thelubricant is reduced.

Patent Document 1: JP 7-268375 A

Patent Document 2: JP 2000-63869 A

DISCLOSURE OF THE INVENTION Problems to be solved by the Invention

The present invention has been made in view of such circumstances, andan object of the present invention is to provide a lubricant compositionhaving low viscosity and a long metal fatigue life, while beingexcellent in low-temperature fluidity and oxidation stability, and henceis particularly suitable as a lubricant for automobile transmissions.

Means for Solving the Problems

The inventors of the present invention have made extensive studies witha view to developing a lubricant composition having the foregoingexcellent performance. As a result, the inventors have found that theobject can be achieved by blending a base oil having specific propertieswith an ethylene/α-olefin copolymer having a molecular weight in aspecific range at a predetermined ratio. The present invention has beencompleted on the basis of such finding.

That is, the present invention provides:

(1) a lubricant composition including: a lubricant base oil; and 0.5 to10 mass % of (A) an ethylene/α-olefin copolymer having a number averagemolecular weight of 2,800 to 8,000, in which the lubricant base oil usedhere has a kinematic viscosity at 100° C. of 1.5 to 40 mm²/s, aviscosity index of 100 or more, a pour point of −25° C. or lower, and asulfur content of 0.01 mass % or less;(2) the lubricant composition according to the item (1), furtherincluding 0.01 to 2.0 mass % of (B) a phosphorus-based extreme pressureagent and/or a sulfur-based extreme pressure agent, the component (B)having a total sulfur content of 0.15 mass % or less;(3) the lubricant composition according to the item (1) or (2), furtherincluding (C) at least one kind of an additive selected from anantioxidant, another extreme pressure agent, a wear-resisting agent, anoiliness agent, a detergent, a dispersant, and a pour point depressant;and(4) the lubricant composition according to any one of the items (1) to(3), in which the lubricant composition is used as a lubricant forautomobile transmissions.

According to the present invention, there can be provided a lubricantcomposition having low viscosity and a long metal fatigue life, whilebeing excellent in low-temperature fluidity and oxidation stability, andhence is particularly suitable as a lubricant for automobiletransmissions or the like.

BEST MODE FOR CARRYING OUT THE INVENTION

In a lubricant composition of the present invention, there is used abase oil having the following properties.

The base oil must have a kinematic viscosity at 100° C. in the range of1.5 to 40 mm²/s. When the kinematic viscosity is 1.5 mm²/s or more, theevaporation loss of the lubricant composition is small, so an improvingeffect on fuel efficiency can be obtained. Meanwhile, when the kinematicviscosity is 40 mm²/s or less, a power loss due to the viscousresistance of the lubricant composition does not become very large, sothe improving effect on fuel efficiency can be similarly obtained. Thekinematic viscosity is preferably 2 to 25 mm²/s, or particularlypreferably 2 to 10 mm²/s.

The base oil must have a viscosity index of 100 or more and a pour pointof −25° C. or lower. When the viscosity index is 100 or more, a changein viscosity of the lubricant composition due to a temperature change issmall. The viscosity index is preferably 105 or more, or more preferably110 or more. In addition, when the pour point is −25° C. or lower, thelubricant composition to be obtained has sufficient fluidity even in alow-temperature environment. The pour point is preferably −30° C. orlower, or more preferably −40° C. or lower.

It should be noted that the kinematic viscosity and the viscosity indexare each a value measured in accordance with JIS K 2283, and the pourpoint is a value measured in accordance with JIS K 2265.

In addition, the base oil must have a sulfur content of 0.01 mass % orless. When the sulfur content is 0.01 mass % or less, the oxidationstability of the lubricant composition to be obtained becomes good. Itshould be noted that the sulfur content is a value measured inaccordance with JIS K 2541.

The kind of the base oil is not particularly limited, and each of amineral oil and a synthetic oil can be used as the base oil. Here, anyone of the various conventionally known mineral oils can be used as themineral oil, and examples of the mineral oil include a paraffin basemineral oil, an intermediate base mineral oil, and a naphthene basemineral oil. Specific examples of the mineral oil include a lightneutral oil, medium neutral oil, heavy neutral oil, or bright stockobtained by solvent refining, hydrogenation refining, or the like, and amineral oil obtained by the isomerization of a wax.

In addition, any one of the various conventionally known synthetic oilscan be also used as the synthetic oil, and examples of the synthetic oilinclude poly α-olefin, polybutene, polyol ester, dibasic acid ester,phosphate, polyphenyl ether, alkyl benzene, alkyl naphthalene,polyoxyalkylene glycol, neopentyl glycol, silicone oil, trimethylolpropane, and pentaerythritol, and further include hindered ester. Thosebase oils may be used alone or in combination of two or more kinds. Amineral oil and a synthetic oil may be used in combination.

The lubricant composition of the present invention has the feature ofcontaining the base oil having the foregoing properties and 0.5 to 10mass % of (A) an ethylene/α-olefin copolymer having a number averagemolecular weight of 2,800 to 8,000.

The ethylene/α-olefin copolymer to be used as the component (A) in thelubricant composition of the present invention has a number averagemolecular weight in the range of 2,800 to 8,000. When the number averagemolecular weight is −2,800 or more, an improving effect on the viscosityindex of the lubricant composition to be obtained is exerted. Meanwhile,when the number average molecular weight is 8,000 or less, the shearstability of the lubricant composition to be obtained becomes good. Thenumber average molecular weight is preferably 3,000 to 7,000 from theviewpoints of the improving effect on the viscosity index and theimpartment of good shear stability.

It should be noted that the number average molecular weight is a valuemeasured by vapor pressure osmometry.

As the α-olefin constituting the ethylene/α-olefin copolymer, anethylene/α-olefin copolymer having carbon atoms of preferably 3 to 20,or more preferably 3 to 10 is used. Examples of the α-olefin includepropylene, 1-butene, isobutene, 1-pentene, 1-hexene, 1-octene, and1-decene. One kind of the α-olefin may be used or two or more kinds ofthe α-olefins may be used in combination. Particularly suitable ispropylene in terms of the performance of the composition.

In addition, the content of an ethylene unit in the ethylene/α-olefincopolymer is preferably 15 to 85 mol %, or more preferably 20 to 80 mol% in terms of the performance of the composition. The manner in whichthe ethylene unit and the α-olefin are copolymerized is not particularlylimited, and the copolymer may be either a random copolymer or a blockcopolymer.

In the lubricant composition of the present invention, one kind of theethylene/α-olefin copolymer may be used as the component (A), or two ormore kinds of the ethylene/α-olefin copolymers may be used incombination as the component (A). The component (A) is a component to beblended in to the base oil for providing the lubricant composition ofthe present invention with low viscosity, excellent low-temperaturefluidity and oxidation stability, and a long metal fatigue life.

Therefore, the content of the component (A) is 0.5 to 10 mass %,preferably 0.5 to 6 mass %, or more preferably 0.5 to 4.5 mass % withrespect to the total amount of the composition from the viewpoints ofthe foregoing properties.

A phosphorus-based extreme pressure agent and/or a sulfur-based extremepressure agent can be further incorporated as a component (B) into thelubricant composition of the present invention.

Examples of the phosphorus-based extreme pressure agent includephosphate, acidic phosphate, acidic phosphate amine salt, phosphite,acidic phosphite, and acidic phosphite amine salt.

As the triphosphate, there are given triaryl phosphate, trialkylphosphate, trialkylaryl phosphate, triarylalkyl phosphate, andtrialkenyl phosphate, and examples thereof include triphenyl phosphate,tricresyl phosphate, benzyldiphenyl phosphate, ethyldiphenyl phosphate,tributyl phosphate, ethyldibutyl phosphate, cresyldiphenyl phosphate,dicresylphenyl phosphate, ethylphenyldiphenyl phosphate,di(ethylphenyl)phenyl phosphate, propylphenyldiphenyl phosphate,di(propylphenyl)phenyl phosphate, triethylphenyl phosphate,tripropylphenyl phosphate, butylphenyldiphenyl phosphate,di(butylphenyl)phenyl phosphate, tributylphenyl phosphate, trihexylphosphate, tri(2-ethylhexyl)phosphate, tridecyl phosphate, trilaurylphosphate, trimyristyl phosphate, tripalmityl phosphate, tristearylphosphate, and trioleyl phosphate.

Examples of the acidic phosphate include di-2-ethylhexyl acid phosphate,didecylacid phosphate, didodecyl acid phosphate (dilauryl acidphosphate), tridecyl acid phosphate, dioctadecyl acid phosphate(distearyl acid phosphate), and di-9-octadecenylacidphosphate(dioleylacidphosphate). Examples of the phosphite include triethylphosphite, tributyl phosphite, triphenyl phosphite, tricresyl phosphite,tri(nonylphenyl)phosphite, tri(2-ethylhexyl)phosphite, tridecylphosphite, trilauryl phosphite, triisooctyl phosphite, diphenylisodecylphosphite, tristearyl phosphite, and trioleyl phosphite.

Examples of the acidic phosphite include di-2-ethylhexyl hydrogenphosphite, didecyl hydrogen phosphite, didodecyl hydrogen phosphite(dilauryl hydrogen phosphite), dioctadecyl hydrogen phosphite (distearylhydrogen phosphite), di-9-octadecenyl hydrogen phosphite (dioleylhydrogen phosphite), and diphenyl hydrogen phosphite.

As the acidic phosphate amine salt and the acidic phosphite amine salt,salts formed of the above-mentioned acidic phosphates and acidicphosphites and the following amines are exemplified. As the amines, amonosubstituted amine, a disubstituted amine, or a trisubstituted aminemay be used.

Examples of the monosubstituted amine include butylamine, pentylamine,hexylamine, cyclohexylamine, octylamine, laurylamine, stearylamine,oleylamine, andbenzylamine. Examples of the disubstituted amine includedibutylamine, dipentylamine, dihexylamine, dicyclohexylamine,dioctylamine, dilaurylamine, distearylamine, dioleylamine,dibenzylamine, stearyl monoethanolamine, decyl monoethanolamine, hexylmonopropanolamine, benzyl monoethanolamine, phenyl monoethanolamine, andtolyl monopropanol. Examples of the trisubstituted amine includetributylamine, tripentylamine, trihexylamine, tricyclohexylamine,trioctylamine, trilaurylamine, tristearylamine, trioleylamine,tribenzylamine, dioleyl monoethanolamine, dilauryl monopropanolamine,dioctyl monoethanolamine, dihexyl monopropanolamine, dibutylpropanolamine, oleyl diethanolamine, stearyl dipropanolamine, lauryldiethanolamine, octyl dipropanolamine, butyl diethanolamine, benzyldiethanolamine, phenyl diethanolamine, tolyl dipropanolamine, xylyldiethanolamine, triethanolamine, and tripropanolamine.

In addition, as the acidic phosphate amine salt, a salt formed of theacidic monophosphate and the above-mentioned amine such as monomethylhydrogen phosphate, monoethyl hydrogen phosphate, monopropyl hydrogenphosphate, monobutyl hydrogen phosphate, or mono-2-ethylhexyl hydrogenphosphate can be used.

In the present invention, one kind of the phosphate-based compound maybe used or two or more kinds of the phosphate-based compounds may beused in combination.

Meanwhile, the sulfur-based extreme pressure agent has only to have thefollowing characteristics: the agent has a sulfur atom in any one of itsmolecules, and is dissolved or uniformly dispersed in the lubricant baseoil so as to be capable of exerting extreme-pressure property or anexcellent friction characteristic. Examples of the sulfur-based extremepressure agent having such characteristics include sulfurized fats andoils, a sulfurized fatty acid, a sulfurized ester, a sulfurized olefin,dihydrocarbyl polysulfide, a thiadiazole compound, a thiophosphoric acidester (thiophosphite or thiophosphate), an alkylthiocarbamoyl compound,a thiocarbamate compound, a thioterpene compound, and a dialkylthiodipropionate compound. Here, the sulfurized fats and oils can beobtained by causing sulfur or a sulfur-containing compound and fats andoils (such as a lard oil, a whale oil, a vegetable oil, and a fish oil)to react with each other, and the sulfur content of each of thesulfurized fats and oils, which is not particularly limited, is suitably5 to 30 mass % in ordinary cases. Specific examples of such fats andoils include a sulfurized lard, a sulfurized rapeseed oil, a sulfurizedcastor oil, a sulfurized soybean oil, and a sulfurized rice bran oil.The sulfurized fatty acid is, for example, a sulfurized oleic acid.Examples of the sulfurized ester include sulfurized methyl oleate andsulfurized rice bran fatty acid octyl.

The sulfurized olefin is, for example, a compound represented by thefollowing general formula (I):

R¹—S_(q)—R²  (I)

where: R¹ represents an alkenyl group having 2 to 15 carbon atoms; R²represents an alkyl or alkenyl group having 2 to 15 carbon atoms; and qrepresents an integer of 1 to 8.

The compound is obtained by causing an olefin having 2 to 15 carbonatoms or a dimer, trimer, or tetramer of the olefin and a sulfurizingagent such as sulfur or sulfur chloride to react with each other, andthe olefin is preferably, for example, propylene, isobutene, ordiisobutene.

The dihydrocarvyl polysulfide is, for example, a compound represented bythe following general formula (II):

R³—S_(r)—R⁴  (II)

where R³ and R⁴ each represent an alkyl group or a cyclic alkyl grouphaving 1 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms,an alkyl aryl group having 7 to 20 carbon atoms, or an aryl alkyl grouphaving 7 to 20 carbon atoms, which may be identical to or different fromeach other; and r represents an integer of 1 to 8.when both R³ and R⁴ are alkyl groups, the compound is referred to assulfurized alkyl.

Examples of the dihydrocarbyl polysulfide represented by the generalformula (II) preferably include dibenzyl polysulfide, various dinonylpolysulfides, various didodecyl polysulfides, various dibutylpolysulfides, various dioctyl polysulfides, diphenyl polysulfide, anddicyclohexyl polysulfide.

Examples of the thiadiazole compound preferably include

-   2,5-bis(n-hexyldithio)-1,3,4-thiadiazole,-   2,5-bis(n-octyldithio)-1,3,4-thiadiazole,-   2,5-bis(n-nonyldithio)-1,3,4-thiadiazole,-   2,5-bis(1,1,3,3-tetramethylbutyldithio)-1,3,4-thiadiazole,-   3,5-bis(n-hexyldithio)-1,2,4-thiadiazole,-   3,6-bis(n-octyldithio)-1,2,4-thiadiazole,-   3,5-bis(n-nonyldithio)-1,2,4-thiadiazole,-   3,5-bis(1,1,3,3-tetramethylbutyldithio)-1,2,4-thiadiazole,-   4,5-bis(n-octyldithio)-1,3,4-thiadiazole,-   4,5-bis(n-nonyldithio)-1,2,3-thiadiazole, and-   4,5-bis(1,1,3,3-tetramethylbutyldithio)-1,2,3-thiadiazole.

Examples of the thiophosphoric acid ester include alkyltrithiophosphite, aryl or alkyl aryl thiophosphate, and zinc dilauryldithiophosphate. Particularly preferred are lauryl thiophosphite andtriphenyl tiophosphate.

Examples of the alkyl thiocarbamoyl compound include bis(dimethylthiocarbamoyl)monosulfide, bis(dibutyl thiocarbamoyl)monosulfide,bis(dimethyl thiocarbamoyl)disulfide, bis(dibutylthiocarbamoyl)disulfide, bis(diamyl thiocarbamoyl)disulfide, andbis(dioctyl thiocarbamoyl)disulfide.

Further, the thiocarbamate compound is, for example, a zincdialkyldithiocarbamate, the thioterpene compound is, for example, aproduct of a reaction between phosphorus pentasulfide and pinene, andexamples of the dialkyl thiodipropionate compound include dilaurylthiodipropionate and distearyl thiodipropionate. Of those, thethiadiazole compound or benzyl sulfide is suitable in terms of, forexample, extreme-pressure property, a friction characteristic, andthermal oxidation stability.

One kind of those sulfur-based extreme pressure agents may be usedalone, or two or more kinds of them may be used in combination.

In the lubricant composition of the present invention, only thephosphorus-based extreme pressure agent may be used as the component(B), only the sulfur-based extreme pressure agent may be used as thecomponent (B), or the phosphorus-based extreme pressure agent and thesulfur-based extreme pressure agent may be used in combination as thecomponent (B).

The content of the component (B) is typically about 0.01 to 2.0 mass %,or preferably 0.05 to 1.5 mass % with respect to the total amount of thelubricant composition in terms of, for example, a balance between theeffect and economical efficiency of the component as an extreme pressureagent.

It should be noted that, when the sulfur-based extreme pressure agent isused, from the viewpoint of corrosion prevention, the amount in whichthe agent is blended is preferably adjusted so that the total sulfurcontent in the composition may be preferably 0.15 mass % or less, ormore preferably 0.10 mass % or less.

At least one kind of an additive selected from an antioxidant, anotherextreme pressure agent, a wear-resisting agent, an oiliness agent, adetergent dispersant, and a pour point depressant can be furtherincorporated as a component (C) into the lubricant composition of thepresent invention.

Examples of the antioxidants include an amine-based antioxidants,phenol-based antioxidants, and a sulfur-based antioxidants.

Examples of the amine-based antioxidants include:monoalkyldiphenylamine-based compounds such as monooctyldiphenylamineand monononyldiphenylamine; dialkyldiphenylamine-based compounds such as4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine,4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine,4,4′-dioctyldiphenylamine, and 4,4′-dinonyldiphenylamine;polyalkyldiphenylamine-based compounds such as tetrabutyldiphenylamine,tetrahexyldiphenylamine, tetraoctyldiphenylamine, andtetranonyldiphenylamine; and naphthylamine-based compounds such asα-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine,pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine,heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, andnonylphenyl-α-naphthylamine. Of those, dialkyldiphenylamine-basedcompounds are preferable.

Examples of the phenol-based antioxidant include monophenol-basedcompounds such as 2,6-di-tert-butyl-4-methylphenol and2,6-di-tert-butyl-4-ethylphenol; and diphenol-based compounds such as4,4′-methylenebis(2,6-di-tert-butylphenol) and2,2′-methylenebis(4-ethyl-6-tert-butylphenol).

Examples of the sulfur-based antioxidant include phenothiazine,pentaerythritol-tetrakis(3-laurylthiopropionate),bis(3,5-tert-butyl-4-hydroxybenzyl)sulfide,thiodiethylenebis(3-(3,5-di-tert-butyl-4-hydroxyphenyl))propionate, and2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazine-2-methylamino)phenol.

Each of these antioxidants may be used alone or two or more of them maybe used in combination. Further, the blending amounts of thoseantioxidants are typically in the range of 0.01 to 10 mass %, orpreferably in a range of 0.03 to 5 mass % on the basis of the totalamount of the lubricant composition.

As other extreme pressure agents, the wear-resisting agents, and theoiliness agents, there are exemplified organic metal compounds such aszinc dithiophosphate (ZnDTP), zinc dithiocarbamate (ZnDTC),oxymolybdenum organo phosphorodithioate sulfide (MODTP), andoxymolybdenum dithiocarbamate sulfide (MDTC). The blending amount ofthose is typically 0.05 to 5 mass %, or preferably 0.1 to 3 mass % basedon the total amount of lubricant composition.

Further, mentioned are oiliness agents including aliphatic saturated andunsaturated monocarboxylic acids such as stearic acids and oleic acids;polymerized fatty acids such as dimer acids and hydrogenated dimeracids; hydroxy fatty acids such as ricinoleic acids and12-hydroxystearic acids; aliphatic saturated and unsaturated monohydricalcohols such as lauryl alcohol and oleyl alcohol; aliphatic saturatedand unsaturated monoamines such as stearylamine and oleylamine; andaliphatic saturated andunsaturated monocarboxylic acid amides such aslauric acid amide and oleamide.

The preferable blending amount of those oiliness agents is typically ina range of 0.01 to 10 mass %, or particularly preferably in a range of0.1 to 5 mass % based on the total amount of lubricant composition.

Examples of the detergents and the dispersants include: ashlessdispersants such as succinimides, boron-containing succinimides,benzylamines, boron-containing benzylamines, succinates, and monovalantor bivalent carboxylic amides typified by fatty acids or succinic acids;and metal-based detergents such as neutral metal sulfonate, neutralmetal phenate, neutral metal salicylate, neutral metal phosphonate,basic sulfonate, basic phenate, basic salicylate, over-based sulfonate,over-based salicylate, and over-based phosphonate. The blending amountof those detergent dispersants is typically 0.1 to 20 mass %, orpreferably 0.5 to 10 mass % based on the total amount of lubricantcomposition.

For example, a polymethacrylate having a weight average molecular weightof about 50,000 to 150,000 can be used as the pour point depressant.

An additive except those described above such as a rust inhibitor, ametal deactivator, a defoaming agent, or a surfactant can beincorporated into the lubricant composition of the present invention asdesired.

There are used, for example: as the rust inhibitor, alkenyl succinicacid and partial esters thereof; as the metal corrosion inhibitor,benzotriazole-based, benzimidazole-based, benzothiazole-based, andthiadiazole-based ones; and as the metal deactivator, benzotriazole,benzotriazole derivatives, benzothiazole, benzothiazole derivatives,triazole, triazole derivatives, dithiocarbamate, dithiocarbamatederivatives, imidazole, and imidazole derivatives. There are used, forexample: as the defoaming agent, dimethyl polysiloxane and polyacrylate;and as the surfactant, polyoxyethylene alkylphenyl ether.

The lubricant composition of the present invention has, for example, thefollowing characteristics: the lubricant composition has low viscosity,is excellent in low-temperature fluidity and oxidation stability, andhas a long metal fatigue life. Accordingly, the lubricant composition issuitably used as, for example, an oil for transmissions, power steeringoil, shock absorber oil, or engine oil in automobiles, or a gear oil,hydraulic oil, or bearing oil for automobiles and industries; thelubricant composition is particularly suitable as an oil fortransmissions such as an automatic transmission, a manual transmission,and a continuously variable transmission in automobiles.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to Examples and Comparative Examples, but is not limitedthereto.

It should be noted that the physical properties of a base oil and alubricant composition were determined in accordance with the followingmethods.

<Base Oil and Lubricant Composition> (1) Kinematic Viscosity (40° C.,100° C.)

Measurement was performed in accordance with JIS K 2283.

(2) Viscosity Index (VI)

Measurement was performed in accordance with JIS K 2283.

(3) Sulfur Content

Measurement was performed in accordance with JIS K 2541.

<Base Oil> (4) Pour Point

Measurement was performed in accordance with JIS K 2265.

<Lubricant Composition> (5) Low-Temperature Viscosity (−40° C.)

A Brookfield viscosity at −40° C. was measured in accordance with ASTM D2983.

(6) Fatigue Life

A fatigue life was measured with a four-ball rolling fatigue tester inthe following manner.

(Bearing) Material: bearing steel Test piece: φ60 × 5 mm thick Steelball dimensions: φ⅜ inch (⅜ × 2.54 cm) (Test conditions) Load: 147 NRotational speed: 2,200 rpm Oil temperature: 120° C.

A time period required for flaking to occur in the test piece wasdefined as a fatigue life, and an L50 (average) was calculated from theresults of the test performed six times.

(7) Oxidation Stability

A test oil was forcedly deteriorated in an ISOT test (1500° C.) inaccordance with JIS K 2514. The percentage by which the viscositychanged (40° C., 100° C.), an increase in total acid number, a basenumber, the amount of n-heptane insoluble matter, and an increase incopper element were measured 144 hours after the deterioration.

The kinds of the respective components used in the preparation of alubricant composition are as shown below.

Base Oils A to F

Table 1 shows the properties of the base oils.

TABLE 1 Properties Kinematic Pour Sulfur viscosity (mm²/s) Viscositypoint content Kind 40° C. 100° C. index (° C.) (mass %) Base AHydrogenation-refined 9.79 2.686 113 −42.5 0.01> oil base oil BHydrogenation-refined 7.976 2.252 83 −10.0 0.01> base oil CHydrogenation-refined 20.54 4.412 127 −22.5 0.01> base oil DHydrogenation-refined 408.8 30.86 107 −15.0 0.01> base oil ESolvent-refined 458.6 30.65 96 12.5 0.43 base oil F Solvent-refined745.8 42.52 98 15.0 0.51 base oil

Ethylene/α-Olefin Copolymer I

A product available under the trade name “Lucant HC600” from MitsuiChemicals, the product having a number average molecular weight of2,600, a kinematic viscosity at 100° C. of 600 mm²/s, a viscosity indexof 240, and a sulfur content of less than 0.1 mass %

Ethylene/α-Olefin Copolymer II

A product available under the trade name “Lucant HC2000” from MitsuiChemicals, the product having a number average molecular weight of3,700, a kinematic viscosity at 100° C. of 2,000 mm²/s, a viscosityindex of 300, and a sulfur content of less than 0.1 mass %Ethylene/α-olefin copolymer III (including diluent oil having aconcentration of 50 mass %)

A product available under the trade name “Lucant HS4010M” from MitsuiChemicals, the product having a number average molecular weight of6,600, a kinematic viscosity at 100° C. of 2,000 mm²/s, a viscosityindex of 335, and a sulfur content of less than 0.1 mass %

Viscosity Index Improver I

A polymethacrylate-based product available under the trade name“VISCOPLEX 0-050” from Romax, the product having a number averagemolecular weight of 18,000 and a kinematic viscosity at 100° C. of 450mm²/s

Viscosity Index Improver II

A polymethacrylate-based product available under the trade name “ACLUBEC728” from Sanyo Chemical Industries, Ltd., the product having a numberaverage molecular weight of 45,000 and a kinematic viscosity at 100° C.of 852 mm²/s

Pour Point Depressant

A polymethacrylate-based product available under the trade name “ACLUBEC728” from Sanyo Chemical Industries, Ltd.

Extreme pressure agent: thiadiazole

ATF Additive Package

A product available under the trade name “Infineum T4261” from Infineum,the product containing a phosphorus-based extreme pressure agent, asulfur-based extreme pressure agent, a detergent dispersant, a frictionadjustor, and an antioxidant

Examples 1 to 4 and Comparative Examples 1 to 6

Lubricant compositions each having the composition shown in Table 2 wereprepared, and were each evaluated for physical properties exceptoxidation stability. Table 2 shows the results.

The lubricant compositions of Example 1 and Comparative Example wereeach evaluated for oxidation stability. Table 3 shows the results.

TABLE 2 Comparative Example Example 1 2 3 4 1 Composition Base oil KindA A A A B/C (mass %) Content 84.2 84.2 83.5 84.1 35.7/44.2 Sulfurcontent 0.01> 0.01> 0.01> 0.01> 0.01> (mass %) Ethylene/propylene I — —2.7 — — copolymer II 4.0 — 2.0 4.0 — III — 4.0 — — — Viscosity index I —— — — — improver II — — — — 8.3 Pour point depressant 0.3 0.3 0.3 0.30.3 Extreme pressure agent — — — 0.1 — ATF additive package 11.5 11.511.5 11.5 11.5 Physical Sulfur content (mass %) 0.06 0.06 0.06 0.06 0.06properties Kinematic viscosity  40° C. 25 26.11 26.07 24.95 33.1 (mm²/s)100° C. 5.64 5.806 5.775 5.63 7.37 Viscosity index 176 176 174 176 198Low-temperature viscosity 7,500 7,200 9,200 7,500 14,500 [−40° C.][Brookfield viscosity] Fatigue life [L50] (minutes) 155 160 162 170 95Comparative Example 2 3 4 5 6 Composition Base oil Kind A/C A/C/D A/EA/F A (mass %) Content 19.0/66.2 50.0/25.2/ 69.2/17.0 73.5/14.0 82.410.0 Sulfur content 0.01> 0.01> 0.08 0.08 0.01> (mass %)Ethylene/propylene I — — — — 5.8 copolymer II — — — — — III — — — — —Viscosity index I — — 2.0 — — improver II 3.0 3.0 — 0.7 — Pour pointdepressant 0.3 0.3 0.3 0.3 0.3 Extreme pressure agent — — — — — ATFadditive package 11.5 11.5 11.5 11.5 11.5 Physical Sulfur content (mass%) 0.06 0.06 0.14 0.14 0.06 properties Kinematic viscosity  40° C. 26.4626.61 28.06 27.24 26.37 (mm²/s) 100° C. 5.729 5.682 5.762 5.690 5.828Viscosity index 167 162 153 156 174 Low-temperature viscosity 8,1009,900 17,200 19,200 11,000 [−40° C.] [Brookfield viscosity] Fatigue life[L50] (minutes) 110 135 140 155 140

TABLE 3 Comparative Example 1 Example 5 Oxidation Percentage by which 40° C. 3.0 8.2 stability kinematic viscosity 100° C. 1.5 4.5 [ISOT testchanged (%) 150° C., Increase in total acid number −0.05 0.12 144 h](mgKOH/g) Base number (mgKOH/g) 0.36 0.05 Amount of n-pentane insoluble0.01> 0.05 matter (mass %) Increase in Cu element (ppm) 26 35

As can be seen from Table 2, each of the lubricant compositions of thepresent invention (Examples 1 to 4) has a low-temperature viscosity(−40° C.) of less than 10,000 and a fatigue life [L50] in excess of 150minutes, so each of them has good low-temperature fluidity and a longmetal fatigue life. In contrast, each of the lubricant compositions ofComparative Examples 1 to 4 and Comparative Example 6 has a fatigue life[L50] of less than 150 minutes, so each of them has a short metalfatigue life. Moreover, each of the lubricant compositions ofComparative Examples 1, 4, and 6 has a low-temperature viscosity (−40°C.) in excess of 10,000, so each of them is poor in low-temperaturefluidity.

Meanwhile, the lubricant composition of Comparative Example 5 has afatigue life [L50] in excess of 150 minutes, so the lubricantcomposition has a long metal fatigue life. However, the lubricantcomposition has extremely bad low-temperature fluidity. In addition, ascan be seen from Table 3, the lubricant composition is inferior inoxidation stability to the lubricant composition of Example 1.

INDUSTRIAL APPLICABILITY

The lubricant composition of the present invention has the followingcharacteristics: the lubricant composition has low viscosity, isexcellent in low-temperature fluidity and oxidation stability, and has along metal fatigue life. Accordingly, the lubricant composition issuitably used as, for example, an oil for transmissions, power steeringoil, shock absorber oil, or engine oil in automobiles, or a gear oil,hydraulic oil, or bearing oil for automobiles and industries.

1: A lubricant composition, comprising: a lubricant base oil; and 0.5 to10 mass % of (A) an ethylene/α-olefin copolymer having a number averagemolecular weight of 2,800 to 8,000, wherein said lubricant base oil hasa kinematic viscosity at 100° C. of 1.5 to 40 mm²/s, a viscosity indexof 100 or more, a pour point of −25° C. or lower, and a sulfur contentof 0.01 mass % or less. 2: The lubricant composition according to claim1, further comprising 0.01 to 2.0 mass % of an extreme pressure agent(B) selected from the group consisting of a phosphorus-based extremepressure agent, a sulfur-based extreme pressure agent, and a mixturethereof, the component (B) having a total sulfur content of 0.15 mass %or less. 3: The lubricant composition according to claim 2, furthercomprising (C) at least one kind of an additive selected from the groupconsisting of: an antioxidant; another extreme pressure agent; awear-resisting agent; an oiliness agent; a detergent dispersant; and apour point depressant. 4: A lubricant for automobile transmissionscomprising the lubricant composition according to claim 1.